477 



Pressure signals 



I — I — I — I — I — I 50 scale units (su) 



Cavitation 



Max extent. 



Collapse(«nnax p(Hz! 



Tc/qc^Tg ) 



Tq (ms) 



Tc(ms) 



0.53 



2U 



16 



0.32 



13 



in 



a 



Q. 



IT) 

 U3 

 CM 



in 

 o 





0.75 



10 



u\ 



u 



0.25 





(D 



a. 

 o 



u 



O 



c 



7^> 



29 



1.06 



u 



11 



0.28 



FIGURE 10. Oscillating hydrofoil. Pressure signals and cavitation, a 



3° a = 4° 



(approximately) that moment when maximum pressure 

 is generated. For rapidly collapsing cavities the 

 cavitation patterns shown existed 1/3-2/3 milli- 

 seconds before the sharp pressure pulse. A note 

 is also made as to whether or not the maximum 

 pressure increase coincided with the final collapse 

 (i.e., the complete disappearance of the cavity). 

 The collapse velocity during the last stage is 

 indicated by arrows : 



> = slow motion of the cavity boundary in 

 the direction of the arrow 



>> = fast motion of the cavity boundary in the 

 direction of the arrow 



>>> = very fast motion of the cavity boundary 

 in the direction of the arrow 



At collapses with more or less spherical symmetry, 

 arrows are placed opposite each other. 



To the right is shown the cavity growth time, T„, 

 and the collapse time, T^,, for the complete cavity, 

 measured by use of the time markings on the high- 

 speed film. The collapse time is measured from the 

 time of maximum area extent to that time when the 

 cavity generated the maximum positive pressure. For 

 rapidly collapsing cavities this event coincides 

 with complete disappearance of the cavity. This 

 was not the case for slowly collapsing cavities; 



for these cavities the collapse times for complete 

 disappearance are also given (in parenthesis) . 



General 



The general character of noise and cavitation be- 

 havior when the frequency of oscillation is varied 

 is shown in Figures 9-14. The pressure signals 

 from the cavitating hydrofoil are to be compared 

 with signals from the non-cavitating hydrofoil 

 (Figure 15) and with the curve in Figure 16, showing 

 the schematic behavior of the pressure generated by 

 a growing and collapsing cavity. 



In comparisons of generated pressure from non- 

 cavitating and cavitating hydrofoils the most 

 striking difference is often the high and sharp 

 pulses generated at the cavity collapse. The 

 generation of such pulses is obtained especially 

 when fosc exceeds a certain value. Also the pressure 

 increase corresponding to cavity growth and the 

 pressure dip generated near maximum cavity extent 

 are detectable. 



The generated pressure pulses were classified 

 into three main types: 



